Raoul has covered most of the important points. Converting a stock ET on-orbit is not cost effective. Modifying the ET on the ground for easy on-orbit conversion makes the ET too heavy and expensive for effective use as a fuel tank. As Raoul mentioned, NASA originally considered a "wet Skylab" where the Saturn S-IVb was actually used for propulsion and then converted on-orbit into a space station. The idea was rejected after the engineers got into the fine details of the design. It's very hard to do stuff in a pressure suit. An astonaut's hands are typically black-and-blue from bruising after a long EVA. This is a problem with Mars colonization. One must devise a system where the colonists are not doing heavy labor inside pressure suits. My nutty idea is to place a pressure tent over a work area and then inflate the tent to 0.21 bar with Martian atmosphere from a turbo-compressor (a high leak rate would be assumed). Workmen would enter the tent and then strip down to shorts and t-shirts. They'd breath pure oxygen from face masks attached to backpack O2-bottles (like a fireman's breathing kit except it would be pure oxygen rather than compressed air).

Gary